Western Railroad Discussion > Deck plate vs through plate girder bridges

Pros and cons!

Mike Root
Madras, OR

Deck bridges (girder or truss) are used when clearances underneath are not an issue. Through girders and trusses are used when crossing roads, rivers, or other railroads. When crossing a river, if a bridge is located well above flood stage and the river is not navigable, a deck bridge may be used.

My understanding is that that a deck plate bridge is generally preferred as long as there is sufficient clearance.      Deck type bridges are cheaper initially and for ongoing maintenance.     They also have less of a clearance issue for oversize loads.

Stephen

Good timing, we just presented a study for the replacement of a through girder railroad bridge.

Through girder bridges are fracture critical since the loads are supported on the two through girders. If either fails, the structure fails. A deck grider bridges has multiple girders and therefore built-in redunancy. Maintenance and inspection costs for deck girder bridges are typically lower. 

An engineering study can determine the optimum solution for a specific project.

That's an issue in NJ where a CSX (former New York Central) bridge crosses ovaer a reservoir, Drainage for the new deck bridge is a concern with oil train and other haz-mat activity.

(Of course, in the past 140 years nothing has happened...)

A few more considerations:
Through plate girder bridges (and through trusses) are vulnerable to being struck by a derailed piece of equipment that can dislodge the girder if it is struck hard enough
Some new TPG designs have an end taper to deflect any such impact.
TPG bridges also have cross-beams at right angles to the track then a set of floor  beams parallel with the tracks to carry the load, this means many more connections and more steel to span the same distance.
Either a deck of through design can be open deck (ties resting directly on the beams) or ballast deck (a steel, wood, or concrete deck that supports ballast with the track in the ballast).
Ballast deck bridges are usually more expensive (they have a higher "dead load" but provide better track dynamics (ride) and are more resistant to fire.
Drainage from an open deck bridge falls directly down to the ground, water, or whatever.
A ballast deck bridge can have the downspouts directed to less objectionable locations.

Sounds like no reason to ever use a thru-girder bridge
if there's room underneath for a deck-girder?

A hundred years ago Rwy Age had a short item comparing
the two SP main line crossings of the LA River- as I recall the
thru-girder bridge weighed twice as much as the thru-truss.

Another important consideration is the length of the span required. DPG's are best (efficiency of design, cost effectiveness, long term fatigue performance, etc.) for up to about 50-60', while TPG's can go up to about 150'. Longer than that and you start to look at trusses. 

The above assumes we are only comparing steel structures. Concrete has another set of criteria and uses. 

Why can through girders go longer than deck (effectively/economically)? Expiring minds want to know.

xrds72 Wrote:
-------------------------------------------------------
> Another important consideration is the length of
> the span required. DPG's are best (efficiency of
> design, cost effectiveness, long term fatigue
> performance, etc.) for up to about 50-60', while
> TPG's can go up to about 150'. Longer than that
> and you start to look at trusses. 
>
> The above assumes we are only comparing steel
> structures. Concrete has another set of criteria
> and uses. 

Posted from Android

Nomad Wrote:
-------------------------------------------------------
> Why can through girders go longer than deck
> (effectively/economically)? Expiring minds want to
> know.

Longer (taller) girder webs...



Edited 1 time(s). Last edit at 08/04/15 05:53 by ats90mph.

A ballasted right-of-way is the way to go for a heavy main line.  When UP had its wreck at Shermer, Illinois, a couple of years ago on July 4 (derailing and dropping the bridge on a car and burying its occupants in the coal), I believe the cause was a heat-kink (it was 100 degrees that day) caused by the change of track structure between the ballasted right-of-way and the deck-trestle.

Carl Shaver
Lombard, IL

> Why can through girders go longer than deck
> (effectively/economically)?

Anyone know? Assuming enough room for deck girders
as tall as the thru girders.

I remembered wong about the SP bridges--
the First Crossing of the LA River had three
96-ft thru-girder spans and weighed 100 tons more
than the thru-truss Second Crossing that has
three 113-ft spans. Same load rating.